John O'Callaghan is an Irish DJ, mainly producing electronic music. He also produces music under the alias of Joint Operations Centre, Mannix and Stenna. He has also collaborated with fellow Discover artists such as Bryan Kearney, Neal Scarborough (as Inertia), Thomas Bronzwaer (as Lost Word) and Greg Downey. His best known track is Big Sky, done with vocalist Audrey Gallagher.

O'Callaghan has had the majority of his work released on Discover, the British record label set up by John Askew. Recently, his work has been released by sub-labels of the Armada Music label; primarily on Armind and Soundpiercing. The remix of his vocal trance production "Big Sky" by Agnelli & Nelson was voted “Tune of The Year” by listeners of Armin van Buuren's A State of Trance radio show in 2007.

In addition to numerous single releases, he has released a live album (Discover "Live As" Volume and an artist album, Something To Live For. He entered the DJ Mag 100 in 2008 at number 60.